A device is known in the art which controls drive force and braking force of a vehicle so that the relative distance between a vehicle and a vehicle in front can be maintained at a proper distance. In this device, the braking force is caused to act automatically when the relative distance becomes shorter than the proper distance, and the drive force is increased so as to decrease the relative distance when the relative distance becomes longer than the proper distance.
Tokkai Hei 5-246270 published by the Japanese Patent Office in 1993 discloses technology which immediately discontinues relative distance control when the driver operates the brake pedal or the accelerator pedal so as to reflect the driver's acceleration and deceleration demands.
However, according to this technique, if the driver depresses the accelerator pedal when the braking force is acting, the braking force stops acting and the vehicle suddenly accelerates. Moreover, if the driver depresses the brake pedal when the drive force is increased, the increase of drive force is stopped and the braking force increases, so the vehicle suddenly decelerates.